Frank and Danny are set to join forces in "Blue Bloods" season 6 episode 18. Also, the latter is going to cross paths with the serial killer in the show's episode 20.

"Blue Bloods" season 6 episode 18 is entitled "Town Without Pity." According to Spoilers Guide, Frank and Danny will work together to lend a hand to ex-convict Nick Constantine, who has just been rehabilitated. As per the publication, the ex-convict was also the one whom Frank put away years ago when he got caught in a murder issue.

The publication added that Erin will get herself ready for a murder trial and is going to discover that the prosecutor who previously took charge of the case is taking back the location of the reported killer so she can contact him face to face on her popular podcast.

Meanwhile, "Blue Bloods" season 6 episode 20 is entitled "Down the Rabbit Hole." As per Carter Matt, the episode links to how Danny deals with his mission on stopping a serial killer in Thomas Wilder. According to the publication, the character was seen being challenged recently, adding that the series is going to come to its heights in "Down the Rabbit Hole" episode in April.

According to Spoilers Guide, "Down the Rabbit Hole" is also set to see Wilder attach a message to his latest victim, which taunts Danny and reopens their game of cat and mouse.

"Blue Bloods" season 6 episode 18, "Town Without Pity," is set to air on Apr. 1 while episode 20, "Down the Rabbit Hole," airs on Apr. 15.