Jenna Elfman is bubbly, and if bubbly isn’t your bag, you have no business watching “Accidentally on Purpose.’’ The humdrum CBS sitcom, premiering tonight at 8:30 on Channel 4, is a vehicle for the former star of “Dharma & Greg,’’ where she played a bubbly hippie chick with bubbly marital issues. This time, she’s doing pregnant-cougar-bubbly, featuring lots of bubbly romantic banter with the boy who knocked her up.
And “knocked up’’ is the best phrase, because this series plays a bit like an adaptation of the movie with that title. In fact, though, “Accidentally on Purpose’’ is based on a memoir by Mary F. Pols, a Bay Area film critic in her late 30s who got pregnant during a fling with a guy in his 20s. Elfman plays the pregnant critic Billie, who whimsically decides to let her boy toy, Zack (Jon Foster), move in with her strictly as a roommate. Of course, flirtation ensues, as does generational clash as Zack and his slacker buddies offend Billie’s more adult sensibilities.