No need for a plugin, just use twig's `slice` filter, combined with the `striptags` filter.

I've thrown in the `default` filter for good measure, in case the body is empty for some reason.

    {{ entry.body|striptags|slice(0,100)|default('Read more') }}...

`slice` will simply output the first 100 characters (starting at `0` the first character), including spaces into the template. `striptags` ensures that no html tags will be output.