It would be:
Allow: /plans$
Disallow: /plans/
Entries are assumed to have a trailing wildcard so /plans/ and /plans/* are the same thing. However, this also means that /plans will also match /plansandstuff. This can be dealt with by using $ which matches "end of path".
See also: Robots.txt Specification
Keep in mind that the robots.txt file is advisory and not all crawlers pay attention to it.
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