Sixth grader James and his Mom are at their wits' ends. His usually bubbly younger sister Grace has turned rude, grumbly, and sulky. After Grace storms out of dinner and stomps up the stairs, James realizes that this is a problem beyond him. He decides it's time to turn to the next book in a magical series he received from his neighbor Stan. As he dives into the Troll Bridge, a Troll the size of his room appears, slumps down on his floor and refuses to speak. James can't believe it. Now he has two trolls to deal with!

The troll follows James like a dark rain cloud to the bowling alley, a pet store, and then back to school. As James observes his tough towering shadow, he begins to see signs of kindness.

Can James help bring out the light in "Tar" and help his sister, or will he be stuck living under the same roof with two incredibly grumpy creatures?