What forced Richard Castle to erase his memory?Screenshot/Video

"Castle" season 7 episode 20 titled "Sleeper" will take Richard Castle (Nathan Fillion) and Kate Beckett (Stana Katic) on a roller coaster ride. But fans will have to wait until 20 April to watch it as the ABC series is on a short hiatus.

The sequel will not take viewers through the disappearance of the former mystery writer, but is also expected to introduce some unexpected plot line.

The official synopsis of the upcoming sequel states that a mysterious recurring dream will drive the couple to seek answers about the two-month period when the novelist went missing. But their search for truth has deadly consequences.

Meanwhile, a new trailer hints that Fillion's character may get into danger if he tries to dig up his past. The two-minute, 28-second-long clip begins by showing him walking through a mysterious place trying to recollect his memory.  Within moments, he turns after hearing a voice that says, "You were not suppose to come back ever."

In the next scene, the mysterious person comes alive on screen with a gun in his hand pointing to Richard. Although the protege tries to identify the person, he fails to do so. The mysterious man informs the protege that he never wanted to remember those days and that's why he asked him to erase his memory. 

"Some mysterious are unmeant to be solved. It's over now. Go home, live your life, forget this ever happened. Its what you wanted," says the mystery man. The sneak peek video ends by showing him walking into darkness.

The trailer has triggered the curiosity of viewers to find out the real reason for their favourite character's disappearance. But they will have to wait for few more days to find out the mystery.