"How I Met Your Mother" Season 9, the series finale ("Last Forever") airs Monday, Mar. 31, on CBS. This is the end of Ted Mosby's (Josh Radnor) story. This features a scene filmed with Penny (Lyndsy Fonseca) and Luke (David Henrie) in 2006. HIMYM fans get the last batch of answers to some burning questions.

RELATED: 'How I Met Your Mother' Season 9: 10 of Best Moments, Final Five Episodes; Is There a Funeral in Series Finale? [VIDEO]

So, why did Ted Mosby begin telling the story of how his kids met their mother? One fan theory says the mother is dead. Another says Ted is dead. A third one says Robin is dead. And a fourth one, among many others, it is "Lost" all over again.

"Insane" and "crazy" were the words Cristin Milioti used in describing fan theories. Neither is a confirmation or denial. This is the end of an era for the avid HIMYM fans who have watched every episode of the hit CBS sitcom.

RELATED: 'How I Met Your Mother' Season 9: Creators 'No Comment' on 'Mother is Dead' Theory, Tease Pineapple Closure Before Series Finale [VIDEOS]

Watch the HIMYM series finale preview below or go to this YouTube page. (Spoiler alert!) See the gang in their MacLaren's booth in a never before seen "pilot" clip.

In this Entertainment Weekly report, Josh Radnor refuses to dish major spoilers. But he hints of surprise treats spanning decades. There is a lot to learn, and a sweet closure is possible. Echoing series creators Craig Thomas and Carter Bays, Radnor is confident the fans will enjoy the How I Met Your Mother series finale.

The HIMYM finale is "complicated," says Neil Patrick Harris. Cobie Smulders seemed like she was punched in the gut when she learned of the ending. She said she just had to leave Thomas and Bays right after hearing the story. Alyson Hannigan says there will be a million answers.

Jason Segel was the first to finish filming among the sitcom cast. Reports say the cast celebrated with pizza together with "Marshpillow" after the last shooting day. Many fans are meeting the How I Met Your Mother finale in tears before it even begins. For others, it is simply just about time.

Here is Ted Mosby's best "farewell quote" (in a dialogue with Robin Scherbatsky) in How I Met Your Mother, so far: "In the end, all we can do is promise to love each other with everything we've got. Because love's the best thing we do."

UPDATE: 'How I Met Your Mother' Series Finale Review: 15 Major Spoilers, Ted Mosby's Misdirection Revealed [Watch Promo Video]

Will "How I Met Your Dad" from the same creative minds elicit the same kinds of emotions from the fans? The new sitcom is not likely to have a Barney Stinson. But it is likely to produce a million quotes.

WATCH: How I Met Your Mother Season 9, Series Finale Preview ('Last Forever' Sneak Peek Video)

How I Met Your Mother Season 9, Episode 23, titled "Last Forever" airs on CBS Monday night in the U.S. Fans who do not want any spoilers should stay out of social media sites.

More TV Guide and Feature Spoilers

'How I Met Your Mother' Season 9, Episode 15: Robin Meets Cristin Milioti? NPH Gets Finale Details from Creators [WATCH]
'Revenge' Season 3, Episode 17 Spoilers: More Complications, Daniel Grayson vs Emily Thorne in 'Addiction' [Watch Promo Video]
'Glee' Season 5, Episode 13 Review, Spoilers; Naya Rivera Says She's Not Leaving, What Happens Next in New York? [Watch Videos]