We are now firmly into the May sweeps, and that means networks are trying to rock it with specials, guest stars, finales and, of course, crossovers. NBC lit the drama match last night on Chicago Fire (1.5/5) with the first part of a three-part crossover with fellow Dick Wolf series Chicago P.D. and Law & Order: SVU, and its second such event this season. With SVU’s Mariska Hargitay on board, Chicago Fire edged ahead of CBS’ Person Of Interest (1.4/4), which dipped 7% from last week among adults 18-49, and a special Robin Roberts-hosted 20/20 – Captive: A Journey Of Hope And Survival (1.4/4) interview with the Cleveland kidnapping survivors, which was up 39% from last week’s Forever to win its 10 PM slot. However, in terms of getting heat, Chicago Fire was muted with a 6% dip from last week’s show.

Chicago Fire’s results likely had something to do with the weak lead-in it had in the finale of One Big Happy (0.9/3). The struggling and unlikely-to-return freshman comedy fell 18% from its April 21 show to hit a new low in preliminary ratings. In fact, Chicago Fire saw the smallest decline week-to-week of any NBC show last night with Undateable (1.2/4) and The Voice (2.1/8) down 14% and 19%, respectively.

CENTER: PETA MURGATROYD, VAL CHMERKOVSKIYPart of the bad note hit by The Voice was due to increased competition from the Dancing With The Stars 10th Anniversary Special (1.7/6). With contestants from seasons past, a clip trip over the show’s 20 cycles and a monster opening routine with more than 45 performers, the DWTS special easily topped last week’s slot average of 1.3 for the Fresh Off The Boat and Repeat After Me finales by 30%. While hitting the best result among the demo in the slot since February 10, the DWTS special was down 15% from Monday’s regularly scheduled airing of the show.

With 9 PM’s Marvel’s Agents Of S.H.I.E.L.D (1.4/4) falling 13% from its April 21 show to a new low in preliminary ratings, despite a clear connecting line to the about-to-open The Avengers: Age Of Ultron and a poignant cameo from the greater Marvel Universe, the CW had the top superhero show of the night. With just a three-week dash left until its first season reaches the finish line, 8 PM’s The Flash (1.5/5) jumped up 15% from last week to hit its best demo and viewership (3.84 million) result since February 27. That’s also the first time the freshman WBTV show based on the DC Comics character has topped S.H.I.E.L.D. among 18-49s since its very strong October 7 premiere. The Flash’s strong performance last night proved a good lead-in for iZombie (0.7/2), which was up 17% from last week.

After all that, the winner of Tuesday night was CBS with a 1.7/6 rating and 12.44 million viewers. Its NCIS (1.9/7) was the second-highest-rated show of the night after The Voice but tops in viewership with 14.54 million watching. The Mark Harmon-starrer was steady with last week with a 5% dip. NCIS: New Orleans (1.8/6) was even with its last original April 14.

Fox had a mixed night with Hell’s Kitchen (1.1/4) dipping 8% from last week. The net’s comedies went the other way with New Girl (1.0/3) rising 11% from its last original of April 7 and newbie Weird Loners (0.6/2) making a 20% jump from last week.