NBC is airing an unusual hat-trick series crossover.

Episodes of three NBC dramas, "Chicago Fire," ''Law & Order: Special Victims Unit" and "Chicago P.D.," will share a story line about an effort to crack a child pornography ring.

The three-parter begins at 10 p.m. EST Tuesday on "Chicago Fire" and wraps up Wednesday on the two other NBC dramas airing back to back from 9-11 p.m. EST.

Making the ambitious crossover feasible: All the shows are from veteran producer Dick Wolf and Universal Television.

[Associated Press]