We want to preface this article by saying that this is only a theory we are speculating on about How I Met Your Mother. Again, pure speculation. We have no idea how the series is going to end; we are only basing this theory on clues and how we read into certain episodes of the CBS comedy.
The theory we’re talking about is that the mother is dead, and that’s why Ted (Josh Radnor) has been telling his kids this long story about how he met her. Either she died recently and Ted is reminiscing with his children or she died before the kids were old enough to remember her and he’s telling them about the mother they never really knew.
Source: E online
