Dennis's birthday is coming up and he is hoping for a one specific present, the XJ3000 Robo Security Shark to guard his treehouse. The only problem is that his relatives are on strike present-wise as Dennis has never actually thanked them for any of his presents. In an attempt to make amends, Dennis decides to write out thank-you letters, one for every birthday and Christmas. However, Dennis's menacing reputation precedes him, and everyone thinks his letters are booby-trapped.
Dennis tries everything to say thank you to his relatives, but each plan backfires. In the end, Gran comes to his aid and even manages to get him that shark he was after. It was a real shark he wanted, wasn't it?