My girlfriend's mother was going to be spending the weekend with us. She lives out of town and was going to be taking the train in. Her train was going to be arriving around 6:00pm on Friday and my girlfriend asked if I could get off work early so I could go pick her up. My girlfriend said she would try to get off early too but didn't think she could because they were short staffed that day. She said if she got off early it would be last minute so most likely it would just be me picking up her mother. I got off early and when I got home I needed a shower so I stripped down and began to make my way to the shower when I suddendly heard the shower come on. It was about 4:00pm and I figured my girlfriend managed to get off early too. Since we have showered together before I decided to jump into the shower with her. I slightly opened the shower curtain to not let the water run out and jumped in. She was surprised and so was I. It was not my girlfriend but her mother. I asked what she was doing there and she told me she was able get an early train and just made her own way here. She told me my girlfriend told her where the spare key and she let herself in. We were in the shower naked together and her mother suggested since we were already in there that we just make the best of the situation and just shower together. We began to lather ourselves and soon we helped each other soap up those areas we couldn't easily reach. It soon turned to lathering each other up including each others private parts with a lot of time spend on those private parts. Next thing that occurred was we took it to the next level. She bent over and I slipped it in. We were fucking in the shower like two animals now. Once we were done we cleaned ourselves off again. Her mother suggested we keep this from her daughter and I agreed. Her mother did add we probably shouldn't do this again but she thinks I enjoyed it as much as she did and she wouldn't mind if we did do it again. It did happen a few more times and my girlfriend doesn't know anything about it.