The green dress appearances. S6Ep1 (the Mermaid Theory) Ted accidentally mentions it with part of another story. S7Ep21 (Now We're Even) we get the actual story. S9Ep16 (How Your Mother Met Me) mermaid theory launch "they can never take what we remember no matter how imperfect it is." (shereen naser) Unique color theory in this dress masterpiece Amazon.com: Theory Women's N Mermaid Dr, Black, 0 : Clothing, Shoes & Jewelry Amazon.com: Theory Women's N Mermaid Dr, Black, 0 : Clothing, Shoes & Jewelry Theory Sleeveless Mermaid Midi Dress Saks Fifth Avenue