Proof of Friendship is a special Drama CD that is included in the Primo Family Arc limited DVD Boxset. The story focuses around how Giotto a.k.a Vongola Primo and his Guardians came to possess the golden pocket watches that each of them hold.
The story begins with Giotto and his Guardians near the end of a meeting. Ugetsu Asari plays his flute until G tells him to stop while Lampow is sleeping, earning him a smack from G that wakes him up. G lectures Lampow, telling him to be more focused; Knuckle agrees with G, telling Lampow that it's about time he becomes more serious about his work. Lampow lazily says that it was scary and that they should be happy that he stays the way he is since he has done the job anyway, receiving a glaring look from G, scaring Lampow. Ugetsu steps in before this could go any further, telling G that Lampow has properly done his role, but G retorts that Ugetsu was being too soft. Alaudi asks if they were finished, and says that he wants to go back home; Demon Spade also asks permission to leave since he still has another work to do. G, however, tells them to wait to reconfirm the end of the meeting and then turns towards Giotto, who seems to be daydreaming. G calls Giotto again, who snaps back to reality, and assures the Guardians that he's alright and dismisses them.
After being dismissed, Ugetsu comments that it's already noon, so Knuckle suggests that they go for lunch together. Giotto, however, kindly refuses because he has another thing to do. G suggests they all go with him, but Giotto says that he'll be alright by himself and leaves. Demon thinks that Giotto's recent behaviour is unusual for leaving earlier than Alaudi, something that G agreed since he's been going somewhere alone for the past three days. Most of Giotto's Guardians are worried about this, since they have no idea where he's been going, with Giotto refusing to tell them. Lampow comes to the conclusion that Giotto has a girlfriend, earning Lampow another smack from G who scolds him that kid should just shut up. Demon, however, playfully thinks that this might be a possibility and goes further, saying that Giotto might be attacked by someone if he were left alone. The Guardians begin to wonder about this possibility, much to Alaudi's annoyance. Alaudi stated that he had enough and left, ignoring Knuckle who tried to convince him to stay. G finally loses his patience by where the conversation was heading and decides to follow Giotto. As G left, Demon commented that G worried too much and he left for his work, leaving the rest of the Guardians in thought.
When G decided to follow Giotto, he was surprised when he realized that Lampow, Ugetsu, and Knuckle decided to tag along as well. G orders them to go home, but Ugetsu and Knuckle insisted they were worried too, while Lampow says he wanted to kill time and to find out who Giotto's lover was, earning another punch from G; Knuckle, however, convinced G that it was part of their duty as Guardians to look after Giotto, finally convincing him to agree to let them come. The rest of the Guardians trail Giotto who was walking around the town. There, he was immediately approached by a little girl, alarming the Guardians who remembers how mafia these days like to use innocent children without their knowing for their scheme. When the little girl took out something from her pocket, the Guardians became tensed and worried. However, the girl turned out only bring out a piece of candy as form of gratitude for Giotto, much to the Guardians' relief. Next, Giotto stopped in alleyway where he was surprisingly welcomed by group of kittens and dogs. This reminded G of how Giotto was fond of small animals since they were little. They decided to wait in case something happened, but much to Lampow's boredom, Giotto did nothing but play with the kittens and dogs until sunset.
After he finally finished playing with them, Giotto continued his walk around the town where he was once again warmly greeted by the townspeople. Realizing how nice they are, Lampow said that there's nothing dangerous will happen to the Primo within town as peaceful as this. Ugetsu agreed and started to think that it was pointless for them to keep following their boss. However, they then saw Giotto being dragged by drunkards into bar and gave him a wine. G was ready to beat them up, thinking that they intend to harm Giotto, but Knuckle stopped him, pointing that they meant no harm since Giotto happily joined them before going to vegetables and fish shop. The Guardians seem confused by what Giotto is going to do with all the foods he has bought. However, before they could think about this any further, they notice that Giotto had bought some expensive wine with a ribbon, making Lampow even more sure that he bought all these stuff as gifts for his girlfriend and comments that the next shop Giotto would visit would be a flower shop, to which G tells him to shut up. This turns out to be wrong, as Giotto instead enters an empty alley, and soon goes into an antique shop, increasing the curiosity of his watching Guardians.
The Guardians can't see anything clearly, but soon sees a silhouette of another person holding a knife with Giotto seemingly defending himself. The Guardians got panicked and immediately rush towards the shop when someone comes out, thinking that their boss might be in danger, but they instead ended up bumping into each other and falling onto the floor at the entrance of the shop, surprising Giotto who came out from the shop. After making sure that everyone was alright, Giotto explains that the knife was actually meant to open a letter and the supposed silhouette of him defending himself was actually a gesture of him refusing the knife. Giotto reveals he had known they were following him but remain quiet because he thought it's about time. As Giotto says this, Demon and Alaudi arrives. Demon asks did he come late, and Alaudi disinterestedly states that he would leave if his time was being wasted on something stupid, but Giotto assures them that they arrive on time.
After leaving the shop, Giotto takes them to a restaurant and finally reveals that he had been secretly planning for a dinner together, which is the reason why he visited all those shops before, as those shops were the ones preparing the foods. He stated that he had placed invitation letters in each of their quarters, but never thought that they will end up following him. G finally understood that when Giotto refused to each lunch together, it was actually an implication that they'll eat together later. Demon teases G for thinking otherwise, but before G could start an argument, Giotto tells them to enter the restaurant and sit down.
During dinner, Giotto reveals that he went to the antique store to pick up something, which turns out to be seven golden pocket watches with Vongola crest engraved on the lid, which he had asked the craftsman, Talbot, to make as proof of their friendship. He hands the watches to his Guardians, who accept it whole-heartedly, thanking Giotto. The story ends with Giotto and his Guardians had a toast up for their eternal friendship.
- There's a spoiler in this CD:
- Appearance of Talbot before his official appearance in chapter 299 of the manga.