Son of The Mask is about an aspiring cartoonist name Tim Avery and his family dealing with the aftermath of finding a mysterious mask that gives the wearer magical powers and causes him to turn into a manic shape-shifter. Tim's dog, Otis, finds the mask, and Tim decides to wear the mask to a party. At the party he becomes a manic magical-powered shape-shifter. Everyone at the party loves it. They all assume that Tim is trying out something for a new cartoon character he is creating. While in this state, he sleeps with his wife, and nine months later his wife has a baby. The baby has retained some of the powers from the mask because Tim was wearing the mask during conception.
Tim's magical baby attracts the attention of the Norse god Odin. Odin is furious with his son, Loki, for losing the mask in the first place. Loki, the Norse god of mischief, is the rightful owner of the mask and lost the mask a long time ago. After hearing of Tim's magical baby, Loki comes looking for the mask. Tim, having lost the mask during the nine-month period between wearing the mask and his baby's birth, is also looking for the mask so that he can use it to create a new cartoon character.
While both Loki and Tim are looking for the mask, Tim's wife, Tonya, leaves on a business trip, leaving Tim to be a full-time dad for the duration of her trip. While Tonya is gone, Loki shows up looking for the mask. Tim's baby's powers start to take over just as Loki finds what he is looking for. The movie takes a turn to absurd cartoonish fun controlled by special effects.