Four couples remain, but only one can take home the Mirrorball Trophy on the two-part finale of 'Dancing with the Stars'

The season 21 champion will be declared Tuesday

Musical performers Include Ariana Grande, Meghan Trainor, Nick Carter, Andy Grammer

After weeks of competitive dancing, the final four couples advance to the finals of "Dancing with the Stars" tonight (8-10:01 p.m., ET). Alek Skarlatos and Lindsay Arnold, Bindi Irwin and Derek Hough, Carlos PenaVega and Witney Carson, and Nick Carter and Sharna Burgess have one last night of competitive dancing, fighting to win the judges' and America's votes to be crowned the "Dancing with the Stars" champion Tuesday (9-11 p.m., ET) on the ABC Television Network.

Tonight, the finalists will compete in two rounds of dance. In the first round, one judge will help each couple choose the creative elements of each dance, which is a redo to a new song. Judge Carrie Ann Inaba will work alongside Lindsay and Alek, as well as Bindi and Derek. Judge Julianne Hough is with Nick and Sharna. Judge Bruno Tonioli is with Carlos and Witney.

In the second round, in one of the biggest competitive dances of the season, the couples take on a freestyle dance featuring special effects, additional dancers and unexpected surprises. There will be special performances from Meghan Trainor, singing "Like I'm Gonna Lose You," and Who Is Fancy, Ariana Grande and Meghan Trainor performing "Boys Like You." At the end of the night, the couple with the lowest combined viewer and judge votes from the previous two weeks will be eliminated.

In the two-hour season finale Tuesday, the three finalists advance to the final stage of the competition. There will be live dancing and show-stopping performances at the Grove shopping center and in the "DWTS" ballroom. The night kicks off with a high-energy opening number, featuring a reunion of all 13 of this season's couples at the Grove. The finale will be filled with performances from Alexander Jean, Andy Grammer, Chaka Khan, Elle King and Carter. Kim Zolciak returns to the ballroom to dance a samba with Tony Dovolani to "I Dream of Jeannie," which she was never able to perform, along with encore dances from the eliminated celebrities.

The cast of the upcoming "Dancing with the Stars: Live!" "Dance All Night Tour" will perform a dance to Avicii's "Broken Arrows" as a sneak peak of the 90-minute show traveling the country this winter. The three finalists will each dance a repeat dance from this season.

In the last element of competition, the couples will perform a new routine as part of a "24-Hour Fusion Challenge." The remaining couples will fuse two contrasting dance styles that they've performed this season and they'll have less than 24 hours to prepare that dance for judges' points. At the end of the night, the winner of the coveted Mirrorball trophy will be announced.

"DWTS" is hosted by Emmy Award winner host Tom Bergeron and Erin Andrews.